Manchester United have made David De Gea an improved offer in a bid to break their contract stand-off, sources have told ESPN FC.

The goalkeeper, who has entered the final year of his deal, has already rejected a five-year contract worth £350,000-a-week.

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Sources have told ESPN FC that United are reluctant to make De Gea -- their player of the year in four of the last six seasons -- their highest paid player but have returned with an improved offer in the hope the 28-year-old will commit his long-term future to the club. Negotiations are still ongoing.

If fresh terms cannot be agreed, De Gea will be able to negotiate with clubs outside England from January 1 and leave Old Trafford on a free at the end of next season.

Sources have told ESPN FC that United are willing to keep De Gea this summer, although there are concerns that a big money bid from Paris Saint-Germain would be too good to turn down for a player with just 12 months left on his contract.

United are monitoring Atletico Madrid's Jan Oblak, AC Milan's Gianluigi Donnarumma and Andre Onana at Ajax in case they are forced into signing a new No.1. The last time De Gea entered the final year of his contract in 2015, United agreed to sell the Spain international to Real Madrid on deadline day.

Meanwhile, United have been told it will take close to £90 million to sign Harry Maguire from Leicester having seen a bid of £70m rejected. Maguire, who is also wanted by Manchester City, is keen to secure a move to a top-six side after agreeing to stay at the King Power Stadium for another season last summer.

United are also set to host a testimonial for Antonio Valencia next summer. The 33-year-old has signed for LDU Quito in his native Ecuador after leaving Old Trafford after 10 years at the club at the end of last season. United will play LDU in Manchester next summer.

Former midfielder Quinton Fortune is in line to take up a role with the Under-23s following the departure of Ricky Sbragia at the end of the season. South African Fortune spent seven years as a player at the club between 1999 and 2006.